The invention relates to a method for evaluating a measured parameter with a measuring cell, as well as to a measuring arrangement.
The invention relates to an evaluating method in connection with a fiber optic interferometric sensor measuring system for measuring measured parameters such as pressure, temperature, expansion and optical refractive indices. In particular, the simple and precise acquisition of vacuum pressures should be possible.
With measuring systems based on principles of interference, various measured parameters can be measured with high resolution and accuracy. Typical representatives of such measuring systems are such which are based on Fabry-Perot (FP) sensors. These measure the optical path length difference in the so-called Fabry-Perot sensor cavity. This path length difference changes as a function of the physical parameter to be measured and corresponds to the magnitude of the difference of the optical path length of light reflected at the front face of the cavity and that which is reflected at the rear face of the cavity. The optical path length difference is calculated from the product of the index of refraction of the material through which moves the light and the geometric path difference traversed by the light. Accordingly, an optical path length difference can change, for example, if the distance between two diaphragms forming the Fabry-Perot sensor cavity changes as a function of the pressure or this distance varies due to material expansion as a consequence of temperature changes. However, it can also vary, for example, through changes of the optical properties (refractive index) of a material located in the cavity or forming the cavity. Such a measuring system is comprised of a Fabry-Perot cavity which forms the sensor proper, a suitable evaluation unit and a light source. If, as the light source, a broadband or white light source with short coherence wavelength, such as, for example, an incandescent bulb or a white-light diode or Light Emitting Diode (LED) is utilized, this measuring system is referred to as White Light Interferometry (WLI). With WLI it is possible to measure absolutely the optical path length difference in the sensor cavity.
The sensor cavity is connected to the evaluation unit by an optical waveguide. The light from the light source is conducted to the sensor cavity via an optical waveguide. As a function of the optical path length or of the parameter to be measured, the light is modulated in this cavity. The modulated light is subsequently conducted back via the same or a separate second optical waveguide to the evaluation unit and evaluated here. The evaluation can, in principle, be realized in two different ways. For this purpose either an interferometer or a spectrometer is utilized.
Polarization and Fizeau interferometers have essentially become widely used as interferometer-based evaluation methods. The polarization interferometers are described in U.S. Pat. No. 7,259,862 B2 by Duplain, the Fizeau interferometers in U.S. Pat. No. 5,392,117 by Belleville et al. Spectrometer-based evaluation units are described in U.S. Pat. No. 6,078,706, Nau et al., as well as in U.S. Pat. No. 7,099,015 B2 by Melnyk. Details of an evaluation algorithm according to prior art can be found in the publication US 2005/0151975 A1 by Melnyk.
In current evaluation units high-quality spectrometers are employed. Their resolution is better than 1 nm and they use linear sensor arrays with more than 3500 discrete sensor elements (pixels). For each measuring cycle all sensor elements must always be read and digitized. The accumulated data quantity is proportional to the number of sensor elements and consequently also determining for the shortest possible cycle time. In the case of the currently utilized spectrometers this time is 50 ms, which corresponds to a maximal refresh rate of 20 Hz. The unit prices for such spectrometers are high, always far above $1,000.00 (typically $1499.00 to 1899.00, depending on the model).
In terms of their structure, Fizeau and polarization interferometers are largely equivalent as shown schematically in FIGS. 1 and 2. Both require inter alia an optical wedge 30 which must be appropriately precisely manufactured. The structure of a Fizeau interferometer is schematically depicted in FIG. 1 (see also U.S. Pat. No. 5,392,117, Belleville). In the Fizeau interferometer this wedge must be provided with reflection layers. The structure of a polarization interferometer is schematically shown in FIG. 2 (see also U.S. Pat. No. 7,259,862 B2, Duplain). Instead of the reflection layers, the wedge 30 in the polarization interferometer utilizes polarizers. Such a wedge with the necessary layers, respectively polarizers, is complex and therefore expensive in production and has undesirable dispersion effects which affect the resulting interferogram and reduce the attainable measuring accuracy. The optical path length in the wedge is also temperature dependent. This dependence can be (partially) compensated, yet has nevertheless a disadvantageous effect on the attainable accuracy and entails increased complexity and expenditures for the realization.
Both interferometer principles, moreover, only supply relative measurement values, i.e. both must be calibrated during the production in order to yield absolute measurement values. In both principles the measuring range is defined by the wedge and, consequently, is fixed. The greatest measurable optical path length is determined by the greatest thickness and the smallest measurable optical path length by the smallest thickness of the wedge. The attainable resolution, defined by the “slope” of the wedge is also fixed.
The attainable measuring accuracy depends inter alia on the contrast and the signal-to-noise ratio of the measuring signal. These values, in turn, are affected by the modulation depth of the sensor and the length or attenuation of the optical waveguide by which the sensor is connected with the evaluation unit. The modulation depth (ratio of modulated to non-modulated light) is determined by the optical coupling system and the optical properties of the sensor cavity. For the partially transmissive mirrors of the cavity a reflection of approximately 25% is ideal. In practice this can only be achieved under high complexity and expenditure with corresponding optically effective coatings. However, such are not realizable in every case and one is forced to utilize non-coated glass surfaces as mirrors or partially transmissive mirrors. Depending on the material used, these still have each only a reflection of approximately 4%. In such a case a measurement signal is obtained with very poor contrast or a very low signal-to-noise ratio. In this case for the evaluation of the measuring signal very high expenditures must be spent and the attainable accuracy is limited.
A spectrometer-based evaluation unit is described in U.S. Pat. No. 7,099,015 B2 by Melnyk. FIG. 3 shows the schematic structure of a corresponding measuring system. A significant disadvantage of this arrangement proposed here, is the complicated calculation of the measured value via normalization of the measured spectrum, direct Fast Fourier Transformation (FFT), bandpass filters, inverse FFT, adding of the phase and subsequent determination of the measured value from a lookup table. This calculation is described in U.S. Pat. No. 7,099,015 B2 by Melnyk and is typically realized with a Digital Signal Processor (DSP) 32. The measured spectrum must also be normalized which, in addition to the measuring sensor 5, presupposes a second reference sensor 31.
US 2005/0151975 A1 discloses an alternative calculation method. FIG. 4 shows the schematic structure of such a measuring system. The alternative calculation method is based on the correlation of measured spectrum with predetermined and stored theoretical spectra. A significant disadvantage is here the store 34 necessary for the storage of the predetermined spectra as well as the calculating time of the corresponding computing unit 32 necessary for the calculation of the correlation 33. The attainable measuring accuracy, moreover, depends on the number of stored spectra and therewith on the available storage space but also on the available calculating time. If one wishes to cover a greater measuring range from, for example, 10 μm up to 100 μm, and attain a resolution in the subnanometer range of, for example, 0.01 nm, approximately 9000 spectra need to be predetermined and stored if it is assumed that a reference spectrum is required every 10 nm. To find in this case the correct one out of this quantity by means of correlation calls for considerable calculation expenditures. While, as described in the (laid open) patent application US 2005/0151975 A1 by Melnyk, this can be reduced again through additional algorithms, it remains nevertheless immense, especially when high accuracy is the goal. Furthermore, the described simplifications (tracking) for the reduction of the necessary calculating time function only if the measuring signal does not change significantly within one measuring cycle. In the case of relatively larger measurement value changes or signal jumps, the tracking method no longer functions since in this case tracking is no longer even possible. If the measuring instrument is to be employed in a stable regulation circuit, in order to be able to ensure stability, the maximal measuring cycle time or response time of the measuring instrument must in this case be assumed. For the reasons just cited (fast signal change, signal jumps) the tracking has thus no influence on the minimal measuring cycle time.
Evaluation units such as are described in U.S. Pat. No. 7,099,015 B2 by Melnyk or in US 2005/0151975 A1 by Melnyk require high-quality spectrometers. Such must have a resolution better than 1 nm and are correspondingly expensive. The unit prices for such spectrometers are always far higher than $1000 (typically $1499 to $1899, depending on the model). The linear sensor arrays utilized in the spectrometers are comprised of more than 3500 discrete sensor elements (pixels), conventionally 3648 pixels. For each measuring cycle all sensor elements must always be read and digitized. The accumulated data quantity is proportional to the number of sensor elements and consequently also determining for the shortest possible cycle time. In the case of the currently utilized spectrometers this time is 50 ms, which corresponds to a maximal refresh rate of 20 Hz.
In summary, it can be stated that the described spectrometer-based methods are unsuitable for industrial implementation, since they require too many resources (storage, computing power), do not permit fast regulation applications and are too expensive.